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    5G Random Access Procedure:

    ·        Why Random Access Procedure Require:
    §  Used for UE UL Synchronization between UE to Network
    §  Require UL Resources for sending Msg3 or data stored in buffer
    §  UE Initial access from RRC_IDLE
    §  Random Access Procedure is always initiated by UE MAC Layer
    §  RRC Connection and RRC Re-establishment procedure
    §  During Handover Time
    §  During RRC_CONNECTED DL or UL data arrival when UE have UL synchronization
    §  Transition from RRC_Idle to RRC_Active State;
    §  To establish time alignment at Secondary Cell addition;
    §  Request for Other System Information
    And Beam failure recovery.

    ·        How many Types of Random Access Procedure:

    Contention Based and Non-Contention Based

    Random Access Procedure types are similar to LTE procedure.
    Contention Based RAP: - Collision possible on PRACH Transmissions from several UE’s.

    §  Non Contention Based RAP: - Collisions will be avoided as Network allocates special pattern and special PRACH Resources.

    *UE MAC Randomly select a preamble from a 64 preambles broadcast in PDCCH by network. How?
    Preamble Sequence Generation

    In UE MAC Zadoff-chu sequence generation is used for generate preamble and it have a property to convert actual no. into complex no. (The complex no. having different angular values)

    From which frequency domain sequence generated 

    Where LRA = 839 or LRA = 139 depending on the PRACH preamble format define below:

    Long Preamble Features:

    §  A long preamble with 1.25 kHz numerology holds 6 resource blocks in the frequency domain, while a preamble with 5 kHz numerology holds 24 resource blocks.
    §  The Long preambles are based on a sequence length is L = 839
    §  Sub-carrier spacing can be 1.25 Khz or 5 Khz for long preamble
    §  Numerology used for long preambles is different from any other NR broadcast
    §  Origin of long preambles partly from preamble used for LTE
    §  Long preambles can only be used for FR1 frequency bands that are less than 6000 Mhz (6 Ghz)
    §  There are 4 different formats for long preface names format # 0, format # 1, format # 2 and format # 3
    §  NR preamble formats 0 and 1 are similar to LTE preamble formats 0 and 2
    The above 64 preambles are using, have 4 formats and are using in generally Numerology 0 which is similar with 4G. But in 5G we have 4 others Numerologies which are used for higher bands. So total 13 different preambles formats are used in 5G.

    The below table shown FR2 preamble formats:

    Short Preamble Features:
    §  The Short preambles are based on a sequence length L is = 139
    §  The sub-carrier spacing for the short-preamble is aligned with the normal NR sub-carrier spacing i.e. 15 KHz, 30 KHz, 60 KHz and 120 KHz.
    §  Short preambles use subcarrier spacing of the following:
    §  In case of operation below 6 GHz (FR1) of 15 KHz or 30 KHz.
    §  60 Khz or 120 Khz in case of operation in high NR frequency band (FR2).
    §  A short preamble holds 12 resource blocks in the frequency domain regardless of numerology
    §  Short preambles are generally shorter than longer preambles and often feature only a few OFDM symbols.
    §  Short preamble formats are designed such that the end of each OFDM symbol acts as a CP for the next OFDM symbol and the length of the preamble OFDM symbol is equal to the data of the OFDM symbol

    §  In most cases it is therefore possible that multiple preamble transmissions within the same RACH slot (opportunity) are collide in a time. In other words, for short previews there may be multiple RACH opportunities in a single RACH slot in the frequency domain as well as in the time domain.

    §  The 5G NR supports a mix of "A" and "B" formats to enable additional formats such as A1 / B1, A2 / B2, and A3 / B3.
    §  The short Preamble formats are the same except for some short cyclic prefixes for the A and B, B formats.
    §  The preamble formats B2 and B3 are always used in combination with the corresponding A formats (A2 and A3)
    §  Micro-preambles are designed to target small / common cell and indoor deployment scenarios.
    §  Short preambles allow gNB receivers to use the same fast Fourier transform (FFT) for data and random-access premature detection.
    §  These preambles are composed of several small OFDM symbols per second preamble, making them more robust against periodic channels and frequency errors.
    §  The short preambles supports analog beam sweeping during PRACH reception, so that the same preamble can be obtained with different beams at GBB
    *After that UE mac gives this preamble’s to ue’s physical for dispatch via PRACH resources:

    *Now UE physical layer calculate RA_RNTI:
    RA-RNTI= 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id
    Zero Correlation Zone:
    Root Sequence:
    *RRC layer configure the following parameters for RAP:

    - prach-ConfigurationIndex: Provide the set of PRACH occasions for the transmission of the Random Access Preamble
    - preambleReceivedTargetPower: initial Random Access Preamble received power;
    - rsrp-ThresholdSSB: This RSRP threshold is used for the selection of the SSB and corresponding Random Access Preamble and PRACH occasion. If the Random Access procedure is initiated for beam failure recovery, rsrp-ThresholdSSB used for the selection of the SSB within candidateBeamRSList refers to rsrp-ThresholdSSB in BeamFailureRecoveryConfig IE;
    - rsrp-ThresholdCSI-RS: This RSRP threshold is used for the selection of CSI-RS and corresponding Random Access Preamble and PRACH occasion. If the Random Access procedure is initiated for beam failure recovery, rsrp-ThresholdCSI-RS shall be set to a value calculated by multiplying rsrp-ThresholdSSB in BeamFailureRecoveryConfig IE by powerControlOffset as specified in TS 38.214 [6];
    - rsrp-ThresholdSSB-SUL: This RSRP threshold is used for the selection between the NUL carrier and the SUL carrier;
    - candidateBeamRSList: list of reference signals (CSI-RS and/or SSB) is used to identifying the candidate beams for recovery and the associated Random Access parameters;
    - powerControlOffset: a power control offset present between rsrp-ThresholdSSB and rsrp-ThresholdCSI-RS and it’s used when the Random Access procedure is initiated for beam failure recovery.
    - powerRampingStep: the power-ramping factor is used to increase preamble transmit power.
    - powerRampingStepHighPriority: in case of differentiated Random Access procedure the power-ramping factor is used.
    - scalingFactorBI: The scaling factor is used for differentiated Random Access procedure;
    - ra-PreambleIndex: nothing but Random Access Preamble;
    - ra-ssb-OccasionMaskIndex: defines PRACH occasion(s) associated with an SSB in which the MAC entity may transmit a Random Access Preamble (see spec 36.321 subclause 7.4).
    - ra-OccasionList: It defines PRACH occasion’s associated with a CSI-RS in which the MAC entity may transmit a Random Access Preamble;
    - ra-PreambleStartIndex: the starting index of Random Access reamble(s) for on-demand SI request;
    - preambleTransMax: used to count maximum number of Random Access Preamble transmission.
    The UE MAC entity handles the RAP procedure in transport Channels called Random Access Channel
    * When UE is configured with SCG (Secondary Cell Group) than 2 MAC entities configured 1st for MCG (Master Cell Group) and 2nd for SCG.
    *The timers and parameters are used in each MAC entities are configured independently else specified.
    *The Serving Cells, CRNTI, Radio Bearers, Logical CHannels, Upper and Lower layer entities, LCGs, and HARQ entities considered by each MAC entity.
    *If the MAC entity is configured with 1’s or more SCells (Secondary Cells), there are multiple DL-SCH and there may be multiple ULSCH as well as multiple RACH have per MAC entity.
    * And 1’s DLSCH, 0’s or 1’s UL-SCH and 0’s or 1’s RACH for each Secondary Cell
    *Or 1’s DL-SCH, 1’s UL-SCH, and 1’s RACH on the Special Cell.
    ** If MAC entity is not configured with any Secondary Cell, than there is 1’s DL-SCH, 1’s UL-SCH, and 1’s RACH per MAC entity.

    Joni Tyagi

    Author & Editor


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